This invention generally relates to thermal inkjet printing. More particularly, this invention relates to the design of heater resistors within the printhead.
Thermal inkjet printers typically have a printhead mounted on a carriage which traverses back and forth across the width of paper being fed through the printer. The printhead includes a vertical array of nozzles which faces the paper. Ink-filled channels in communication with the nozzles also connect to an ink source such as a reservoir. As ink in the channels is expelled as droplets through the nozzles onto the paper, more ink fills the channels from the reservoir. Bubble-generating heater resistors in the channels near the nozzles are individually addressable by current pulses. These pulses are print commands representative of information to be printed such as video signals from a monitor. Each ink droplet expelled from the nozzles prints a picture element or pixel on the paper.
The current pulses are applied to the heater resistors to momentarily vaporize the ink in the channels into bubbles. The ink droplets are expelled from each nozzle by the growth and then collapse of the bubbles.
The heater resistors, which generate the heat for vaporizing the inks, can be fabricated as a resistive layer on a silicon substrate having a silicon dioxide (SiO.sub.2) layer. These layers together with other layers above the resistive layer form a heating element. The resistive layer can be deposited on the substrate using standard thin-film processing techniques and typically comprises a layer of tantalum aluminum (TaAl) up to several hundred Angstroms (.ANG.) thick.
On the scale of the heater resistor, the shock of the ink bubble collapsing upon the resistive layer is a source of significant mechanical fatigue. The problem of fatigue is aggravated in printers which provide for burst mode operation, in which ink droplets can be formed and expelled over fifty thousand times a second.
In addition to the mechanical shock produced by collapsing bubbles, the resistor is subject to thermal fatigue when it is switched on and off at high frequencies. Thermal fatigue is suspected to aggravate a crack nucleation process, eroding the structural integrity of the resistor. Extended burst-mode operation can additionally cause heat accumulation, compounding the problem of thermal fatigue. The turbulent ink can also be quite corrosive on the resistive layer and subject it to corrosion and erosion.
A conventional technique for protecting the resistive layer is to cover it with one or more passivation layers. For example, a TaAl resistor can be coated with a layer of silicon nitride, silicon carbide or, more commonly, both. In addition, an overcoat of tantalum or other metal is applied over the passivation layers as an additional impact buffer and as a means for evacuating leakage current. These additional layers reduce the intensity of the impact stress wave induced by the collapsing bubble on the resistor to protect it from cavitation damage.
These passivation layers, however, have their drawbacks. For one, there is the additional manufacturing complexity involved. Typically seven film layers are required as opposed to two layers for an unpassivated resistor structure. Correspondingly, five (rather than two) masking steps are required. The increased manufacturing complexity also increases costs and decreases yields on a per wafer basis. A second drawback is that the passivation layers impede the dissipation of heat. The unwanted accumulation of heat can affect ink viscosity significantly, which is a critical variable in determining droplet size and velocity. Furthermore, substantial heat accumulation increases stress levels and thus failure rates of the various layers of the heating element. A third drawback of passivated resistors is that the turn-on voltage varies with passivation thickness. This variation makes it more difficult to determine the proper driving voltage for a given resistor. Driving the resistor with too low a voltage can result in insufficient bubble formation, while driving the resistor with too high a voltage rapidly diminishes resistor life through excessive heating.
One solution to the drawbacks posed by passivation layers is to remove them and increase the thickness of the resistive layer. This approach is described and shown in U.S. Pat. No. 4,931,813, commonly assigned to the present assignee and hereby incorporated by reference. The additional thickness of the resistive layer obviates the need for the passivation layers. The resistor can be constructed to contact fluid in the form of ink or vapor in the form of a thermal bubble in the channel. The resistive layer is homogeneous in that a single material, generally a metal alloy such as TaAl, can be used to form the resistor.
Increasing the thickness of the resistive layer, however, reduces the resistance of the heater resistor because its volume is now greater than before while its resistivity is unchanged. To generate the same heat, the drive current (which generates the pulses) must be increased. Increasing the drive current, in turn, may require a redesign of the printer control circuitry within the printer or printhead.